Photoresponsive supramolecular coordination polyelectrolyte as smart anticounterfeiting inks

Counterfeit goods cause substantial economic losses and pose health risks, spurring demand for advanced anticounterfeiting technologies. Photoluminescence printing is prevalent due to ease of use, high throughput, and tunable optical properties. A range of optical materials (upconversion nanoparticles, organic dyes, quantum dots, MOFs, perovskites, and lanthanide complexes) have been used as taggants. However, key challenges remain: (1) many luminescent inks use organic solvents or toxic ions, limiting use for food/medicine; (2) static optical outputs render authentic information visible under ambient or UV light, reducing security; (3) invasive stimuli (thermal, chemical, mechanical) can damage goods or be inconvenient; and (4) practical inkjet printing often requires complex processing or printer modification when using nanoparticle inks. To address these, the authors developed a water-based, photoresponsive supramolecular coordination polyelectrolyte (SCP) formed by an anionic Eu3+-bis(2,6-pyridinedicarboxylic acid) coordination polymer electrostatically assembled with a cationic diarylethene photoswitch. The diarylethene’s reversible ring-opening/closing modulates spectral overlap with Eu3+ emission to enable photoreversible FRET and on/off Eu3+ luminescence switching under noninvasive UV/visible light, enabling high-security, reversible information encoding compatible with commercial inkjet printers.

The paper surveys luminescent anticounterfeiting materials, including upconversion nanoparticles, organic dyes, quantum dots, MOFs, and perovskites, noting lanthanide complexes’ advantages (distinct spectral fingerprints, large Stokes shifts, long lifetimes). Despite progress, limitations include solvent toxicity, static information visibility, reliance on invasive stimuli, and practical printing constraints requiring complex processing or printer modifications. Stimuli-responsive systems are desirable for higher security, with photochromic units (e.g., diarylethenes) offering reversible, fatigue-resistant switching and clear spatiotemporal control under light. Prior works on photoresponsive systems and lanthanide–photochrome dyads underpin the present strategy to couple a lanthanide donor with a diarylethene acceptor to achieve reversible luminescence control via FRET.

• Synthesis of components: A bis-2,6-pyridinedicarboxylic acid ligand (L) was synthesized (two-step route; characterized by NMR and FTIR). Luminescence titration indicated 2,6-pyridinedicarboxylic acid (DPA):Eu3+ coordination stoichiometry of 3:1. The Eu3+-L trimeric coordination polymer was formed by mixing L and EuCl3 in water at a 1.5:1 molar ratio. FTIR showed the C=O stretch of DPA shifting from 1724 to 1625 cm−1 upon coordination; 1H NMR peaks of L broadened after coordination, confirming polymer formation. Eu3+-L exhibited bright Eu3+ emission in water and solid state; aqueous quantum yield was 23.31%.
• Photoswitch synthesis: A cationic imidazolium-modified diarylethene (open form, OF-1) was synthesized in two steps (72% yield) and fully characterized. UV–Vis and 1H NMR confirmed efficient, reversible open/closed photoisomerization.
• SCP assembly: The anionic Eu3+-L (net −3 per Eu center) was mixed with cationic OF-1 at charge stoichiometry (Eu3+:OF-1 = 1:1.5) in water to form Eu3+-L-OF-1 via electrostatic interactions. Zeta potentials confirmed complexation: Eu3+-L = −19.53 mV; OF-1 = +20.15 mV; Eu3+-L-OF-1 ≈ +1.54 mV (near neutral). DLS showed hydrodynamic radius increase from ~220 nm (Eu3+-L) to ~500 nm (Eu3+-L-OF-1). TEM revealed ~300 nm uniform spheres, supporting supramolecular assembly.
• Photochromism characterization: UV–Vis of Eu3+-L-OF-1 showed an OF-1 band at 294 nm and no absorption >400 nm. UV irradiation (300 nm) decreased 294 nm band and produced new bands at 380 and 596 nm with solution color changing to dark blue; an isosbestic point at 323 nm indicated clean two-state behavior. 1H NMR (DMSO-d6:D2O 4:1) after 300 nm UV (60 min) showed characteristic shifts (e.g., thiophene H1 7.30→6.81 ppm; methyl H2 1.88→2.00 ppm; aromatic Ha 6.98→7.04 ppm; Hc 7.52→7.63 ppm), with integration indicating ~94% conversion to the closed form (CF-1). Visible light (>450 nm) irradiation reversed spectra and color to the open form.
• Photophysics and FRET: Eu3+-L emission (sharp Eu3+ lines at 580, 594, 615, 649, 692 nm; excitation band ~265 nm via DPA antenna) overlaps with CF-1 absorption (500–700 nm), enabling FRET in the closed state. Upon 300 nm UV irradiation, Eu3+ luminescence of Eu3+-L-OF-1 was quenched bi-exponentially to a photostationary state in ~60 s. Emission lifetime decreased from ~1289 µs to ~12 µs, and quantum yield from 15.84% to 0.85%, corresponding to ~98% FRET efficiency. Visible light (>450 nm) restored the original Eu3+ emission. Reversibility/fatigue: After 20 UV/visible cycles, luminescence intensity loss was <4%.
• Stability: The closed form Eu3+-L-CF-1 showed long thermal stability (no ring opening at 60 °C in dark) and slow spontaneous cycloreversion at 25 °C (t1/2 ≈ 376.7 min). Minimal self-switching under sunlight over 90 min.
• Printing demonstrations: Aqueous Eu3+-L-OF-1 (based on OF-1 concentration 2.1 × 10−4 M) was loaded into a commercial inkjet printer (Canon PIXMA ip1180) and printed as QR codes on blue PET films. Patterns were invisible in daylight due to the colorless solution. Under 254 nm UV lamp, bright red Eu3+ emission enabled rapid smartphone scanning before significant OF→CF conversion (low absorbance at 254 nm slows switching). Erasure with 300 nm UV rendered patterns invisible under UV and effectively masked on blue PET in daylight; patterns remained erased under sunlight for one month and were fully restored by visible light (>450 nm). Robust performance persisted over at least 20 erase/write cycles.

• A water-based supramolecular coordination polyelectrolyte (Eu3+-L-OF-1) exhibits light-controlled, fully reversible Eu3+ luminescence on/off switching via diarylethene-mediated FRET.
• Nearly quantitative photoisomerization: ~94% conversion to closed form (CF-1) at photostationary state under 300 nm UV (60 s), with an isosbestic point at 323 nm.
• Strong FRET-mediated quenching in the closed state: Eu3+ lifetime reduced from ~1289 µs to ~12 µs; quantum yield from 15.84% to 0.85%; FRET efficiency ≈ 98%.
• Excellent fatigue resistance: <4% loss in luminescence intensity over 20 UV/visible switching cycles; rapid response (~60 s to photostationary state).
• High stability of the closed form: thermal cycloreversion negligible (no opening at 60 °C in dark); long half-life at 25 °C (t1/2 ≈ 376.7 min); minimal self-switching under sunlight for 90 min.
• Practical inkjet printing of high-resolution QR codes from pure water inks without printer modification; noninvasive, remote light-triggered visible/invisible transformations enabling high-security, multiple-use anticounterfeiting tags.

The study addresses key challenges in anticounterfeiting materials by delivering a purely water-based, noninvasive, and printer-compatible luminescent ink with reversible, high-contrast optical switching. By coupling an emissive Eu3+ coordination polymer with a diarylethene photoswitch through electrostatic assembly, the system achieves conformation-dependent FRET that cleanly toggles Eu3+ emission on and off under UV/visible light. The high FRET efficiency, robust fatigue resistance, and thermal stability ensure repeated authentication cycles without degradation. Importantly, the bistability and slow spontaneous cycloreversion of the diarylethene maintain the encrypted (erased) state under ambient conditions, enhancing security. Demonstrations of smartphone-readable QR codes that become fully invisible under both daylight and UV upon UV erasure and are recoverable with visible light validate the applicability for secure, reversible information encoding in a noninvasive manner, overcoming limitations of toxic solvents, invasive stimuli, and printer incompatibilities.

A photoresponsive supramolecular coordination polyelectrolyte was realized by assembling an anionic Eu3+-bis(2,6-pyridinedicarboxylic acid) coordination polymer with a cationic diarylethene photoswitch. The diarylethene’s reversible isomerization modulates spectral overlap with Eu3+ emission to control FRET, yielding a fully reversible luminescence on/off switch with high efficiency, excellent fatigue resistance, and thermal stability. The water-based SCP functions as a security ink compatible with commercial inkjet printers, enabling high-resolution, multiple-use, noninvasive anticounterfeiting patterns with visible/invisible transformations under UV/visible light. This approach demonstrates a promising class of smart, high-security anticounterfeiting materials operable under green conditions.